The long-term objective of these studies is to characterize surface alterations on cells of patients with acute monoblastic leukemia, so as to develop specific reagents for their detection, and to better comprehend the neoplastic process. A specific surface antigen (AMLSGA) shed from leukemic monoblasts in culture has been partially purified and characterized by agarose gel filtration and DEA cellulose chromatography. The molecular weight of the shed compound is estimated at 350,000 to 450,000 daltons, which may represent an oligomer with subunit molecular weight of approximately 78,000. The isoelectric point of the material is 8.6, possibly due to a high content of sialic acid. Murine heteroantisera produced against this material, as well as rabbit and monkey sera, are capable of detecting leukemic cells in bone marrow of patients with acute myeloblastic leukemia. The technique is sensitive enough so that imminent relapse can be detected several months before the marrow becomes morphologically positive. Monoclonal antisera are being prepared and tested, and surface antigen density studies of marrow cells are being performed, using the fluorescence-activated cell sorter and a new technique of videointensification fluorescence microscopy. These studies will allow earlier detection of relapse and help elucidate the relationship of the appearance of AMLSGA to the leukemic transformation.